Recently, golf balls have been multi-layered in order to impart multi functions and high performances to themselves, and the cover has been multi-layered. However, since resins for the cover material typically have poor rebound characteristics compared with core material of rubber, it is required to reduce the cover thickness as possible compared with the conventional golf balls.
As a method of covering on the core with the cover, there have been a press molding method comprising molding the cover resin into a semi-spherical half-shell in advance, covering the core with the two half-shells, followed by pressure molding under heating; and an injection method comprising holding the core in the mold and injection molding the cover composition directly on the core.
In case of molding the cover having small thickness, particularly not more than 1 mm, it is difficult to mold the cover by the injection molding method, because the gap for injecting cover resin is too small. Therefore, the press molding method is mainly used (Japanese Patent No. 3000918).
In Japanese Patent No. 3000918, a golf ball comprising a core and a cover of two layer structure consisting of inner and outer layers, wherein a color difference ΔE in Lab color space between the inner and outer layers is up to 3 as measured by a calorimeter is disclosed.
Golf ball typically has many dimples on the surface thereof, and the dimple generally has a depth of 0.1 to 0.2 mm. In case of the cover having very small thickness as described above, the cover thickness is small particularly at the bottom of the dimple, and the core, which is the under layer of the cover, may be exposed. Even if the cover is formed, the core shows through the cover at the portion that the cover is too thin, and it is problem that the appearance is degraded. In addition, peeling and crack of the cover occurs, and it is problem that the durability is degraded.
In order to solve the problems, a golf ball that the surface of the core has dimples having the same shape as the dimples of the cover at locations corresponding to the locations of the dimples of the cover has been suggested (Japanese Patent Kokai Publication No. 154034/2003).
In Japanese Patent Kokai Publication No. 154034/2003, a golf ball having many dimples on the surface of the cover, which the surface of the core has dimples having the same shape as the dimples of the cover at locations corresponding to the locations of the dimples of the cover is disclosed. However, at the time of molding the cover, it is difficult to fit projections corresponding to the shape of the dimples in a cover mold, in the dimples on the surface of the core. Therefore, it has been problem to complicate the manufacturing process and take much time.